CEPHALOPODA. 279 



extending parallel with it. It is obscured, towards the central portions of 

 the tube, by an organic deposit on each side of the median line, having a 

 symmetrical form, and presenting the appearance of a sculpturing, or scarring 

 of the ventral walls, and consisting of a palmate arrangement of ridges and 

 nodes of the organic deposit. This ornamentation on the internal mould is 

 finally obliterated near the apex by the predominance of the organic deposit 

 in the air-chambers. The organic deposit is formed over the walls of the 

 air-chambers, and over the surface of the septa, consisting of a vesicular 

 aggregation of calcareous matter, similar in some respects to 0. hxum of the 

 Schoharie grit ; and, as in that species, it increases in amount and com- 

 plexity toward the apex. 



Siphuncle excentric, near the ventral side, straight, cylindrical, slightly 

 constricted in its passage through the septa. The distance from the dorsal 

 and ventral sides are as ten to eighteen, and its diameter is four mm. where 

 the tube has a diameter of thirty mm. 



Test strong, having a thickness of from one to one and a half mm. Sur- 

 face marked by fine, regular, slightly-curving stria) of growth, eleven to 

 seventeen in the space of four mm., crossed by indistinct, irregular, longi- 

 tudinal striae. Towards the apex the longitudinal striae are sharper and 

 stronger, becoming more prominent and regular than the concentric lines, 

 which are here more crowded than in the anterior portions of the tube. A 

 broad, gentle sinus in the direction of the striae, over the siphuncle, indicates 

 the position of the ventral side. 



The internal mould is obscurely, longitudinally striate, and the chamber 

 walls essentially smooth and flat, with the excep'ion of the characteristic 

 ornamentation of the ventral walls. Septal sutures but slightly indented. 



One of the largest individuals has a length of forty centimetres, with a 

 diameter at the aperture of nearly fifty mm., embracing the chamber of habi- 

 tation and about sixty air-chambers. 



This species is quite variable, both in proportions and in general appearance, 

 n- may be seen by consulting the illustrations of the species. The variation 

 in the character of the surface-markings is quite conspicuous on different 



